Computers traditionally use built-in code known as a BIOS (Basic Input/Output System) to perform initial boot functions and to control certain attached devices before loading an operating system or other software. This enables a computer to receive input from a keyboard and to output data to a display, and further provides the computer with control of devices such as serial communications ports and disk drives.
When booting, a personal computer typically either executes the BIOS from ROM or flash memory, or loads the BIOS into RAM and executes the BIOS code from RAM to perform initial testing, configuration, and to initialize loading an operating system. Although operating systems are traditionally loaded from had disks or diskette drives, BIOS extensions such as the PXE or Preboot eXecution Environment provide a computer system the ability to establish a network connection before loading an operating system, and to boot over the network connection via a network bootstrap program. Various other BIOS extensions provide the ability to boot via USB or other devices, but typically require a specially configured bios extension or system BIOS to implement.
The various diskless boot options discussed here require a computer system to have a specialized network interface card or a customized system BIOS, and so may be somewhat difficult to implement in existing systems or with many off-the-shelf parts. In many operating systems the boot disks are assumed to be locally attached storage devices, thus the operating system disk drivers are started first. Later in the operating system startup sequence the network drivers are executed. A boot option enabling a computer system having neither a custom network interface card nor a custom system BIOS is therefore desirable.